Soooo...you joined the Marines. We won't question why for a second, but we'll be back.
They train you to kill people. You are told multiple times that you will be sent out to kill people. You stay through boot camp.
Suddenly, you are called to war and told that you need to kill people like you have been trained.
Oops, suddenly, I'm a "conscientious-objector".
If you are anti-war, at least you are being honest. Misguided, perhaps, but honest.
But for the love of mud, if you are going to join the military, have the intellect to figure out that sometimes, the military will ask you to over-run/neutralize/kill/destroy/secure opposing forces with extreme prejudice.
Our military isn't made for tea-parties. K-bars make horrible stirring sticks. (Don't ask me how I know).